1. Field of the Invention
Example embodiments generally relate to a fixing device, an image forming apparatus including the fixing device, and a fixing method using, for example, induction heating, implemented by a fixing device incorporated in an image forming apparatus.
2. Description of the Related Art
A related-art image forming apparatus, such as a copier, a printer, a facsimile machine, or a multifunction printer having two or more of copying, printing, scanning, and facsimile functions, forms a toner image on a recording medium (e.g., a recording sheet). For example, an electrostatic latent image formed on an image carrier is visualized with toner into a toner image. The toner image is transferred from the image carrier onto a recording sheet. A fixing device applies heat and pressure to the recording sheet bearing the toner image to fix the toner image on the recording sheet by various methods. Such methods include, for example, a heating roller method, a film method, and an induction heating method.
In a fixing device using the heating roller method, a heat generating source (e.g., a halogen lamp) heats a heating roller. The heating roller opposes a pressing roller to form a fixing nip between the heating roller and the pressing roller so as to nip a recording sheet bearing a toner image therebetween. At the fixing nip, the heating roller and the pressing roller apply heat and pressure to the recording sheet bearing the toner image.
In a fixing device using the film method, a film having a thermal capacity smaller than a thermal capacity of the heating roller is used as a heating member for applying heat to a recording sheet bearing a toner image.
In one example of a fixing device using the induction heating method, an induction heating coil wound around a bobbin is provided inside a heating roller. When an electric current is applied to the induction heating coil, an eddy current is generated in the heating roller and the heating roller generates heat.
In the heating roller method, the heating roller is preheated so that the heating roller may be heated quickly. By contrast, in the induction heating method, the heating roller may be heated up to a desired temperature quickly, even when the heating roller is not preheated.
Another example of a fixing device using the induction heating method includes both an induction heater and a heating roller. The induction heater includes an induction heating coil to which a power source applies a high-frequency voltage. The heating roller includes a magnetic heat-generating layer that has a Curie point equivalent to a fixing temperature. When the power source applies a high-frequency voltage to the induction heater, the heat-generating layer generates heat.
For example, a temperature of a ferromagnet included in the heat-generating layer increases quickly until the temperature of the ferromagnet reaches the Curie point. When the temperature of the ferromagnet reaches the Curie point, the heat-generating layer loses its magnetic property. Thus, the temperature of the ferromagnet does not exceed the Curie point and is maintained at a desired temperature. The Curie point of the ferromagnet is equivalent to the fixing temperature. Therefore, the temperature of the ferromagnet is maintained at the fixing temperature.
The advantage of such an arrangement is that the heating roller may be quickly and precisely heated to a desired temperature without a complex controller, while a surface of the heating roller provides a proper release property and heat resistance.
When the heating roller includes a core and a resin layer having thicknesses and shapes different from each other, the core and the resin layer may have thermal capacities different from each other. However, an amount of ferromagnet particles contained in the heating roller may be adjusted to heat the heating roller quickly and to control the temperature of the heating roller more precisely. Moreover, ferromagnet particles lose their magnetic property when the temperature of the ferromagnet particles reaches the Curie point. Therefore, the heating roller may not attract magnetic particles contained in toner, preventing toner offset.
When the magnetic heat-generating layer has the Curie point equivalent to the fixing temperature, heat generation of the heat-generating layer may be controlled at end and center portions of the heat-generating layer in a width direction (e.g., an axial direction) of the heating roller. Alternatively, overheating of the heat-generating layer may be prevented locally.
In yet another example of a fixing device, a magnetic flux adjuster moves in a circumferential direction of a heating roller to prevent temperature increase at both end portions of the heating roller, serving as a fixing member, in a width direction of the heating roller.
However, the magnetic flux adjuster may be displaced from its proper position in the circumferential direction of the heating roller, and as a result, temperatures of the both end portions of the heating roller may not be controlled properly.